Yes, change your ticket ahead of time. I have been using the Pacific Surfliner for a "long, long" time and occasionally over the years jumped on an earlier train without changing tickets. Conductors didn't blink an eye. BUT, no longer. A few months ago I boarded the Surfliner at LAX to SAN on an earlier train, Business Class, mid-week. I hadn't done that for a long time. Conductor nicely and calmly, explained that I should have switched ticket before boarding, (lady across the aisle had done the same thing.). He asked us to call the Amtrak toll free line from the train to do the switch. He said Business Class was not full all the way to SD and it shouldn't be a problem. When we both just had an "Oh no, sad face" at the prospect of calling the toll free number from the train, he said not to worry, took our tickets and said he would take care of it. He returned later, handed us back our tickets and said he had made the switch. Really nice man, and I thanked him profusely. But now I know not to switch trains again without a proper ticket.